Devices for controlling the density of ink applied to the material being printed during the periods when the printing press is accelerating and decelerating are known. Such a device is described in document JP-A-2000255037 from Mitsubishi. The purpose of the device in this document is to maintain the density of the ink constant during the transitional stages when printing speeds change. The speed of the inking rollers is controlled in relation to the printing speed through conversion graphs stored in the memory of the system which compensate for any variation in the ink supply during accelerations or decelerations.
In addition to this, devices for adjusting the quantity of ink provided to the paper by the printing unit in relation to the machine speed and the rate of coverage are known.
A first device of this type is described in document US2002/0073.867 from Heidelberg AG. This document applies to machines equipped with an alternating ink source. Ink supply in relation to printing speed is controlled by varying the contact time between the pick-up roller and the inking ball and by regulating the speed of the inker. The purpose of this system is to compensate for local variations in density due to dispersion of the level of coverage over the machine width as printing speeds change.
The second device of this type is described in JP 2001 328 235 from MITSUBISHI. This document describes a system for controlling the supply of ink in relation to the density of the flat tint read from the paper. Control is applied to the speed of the inking roller and the opening of the adjustment screw. The system is not anticipatory, only corrective.
In addition to this, devices for optimising control of inking during the transitional stages of speed changes are known. Such a device is described in document DE 100 13 876. This document describes an algorithm for self-adaptive adjustment of the supply of ink to a printing machine which anticipates inking corrections through opening the adjustment screw to provide a stabilised speed during transitional regimes. The device calculates these corrections in relation to the difference between the densimetric or calorimetric values measured by a reading system and values calculated from a mathematical model.
The purpose of the device described is to reduce control errors and thus to increase the productivity of printing presses.